Hibernation versus Daily Torpor in Mammals and Birds: Physiological Variables and Classification of Torpor Patterns

Abstract

Hibernation and daily torpor are usually considered to be two distinct patterns of heterothermia. In the present comparison we evaluated (1) whether physiological variables of torpor from 104 avian and mammalian species warrant the distinction between hibernation and daily torpor as two different states of torpor and (2), if so, whether this distinction is best based on maximum torpor bout duration, minimum body temperature (Tb), minimum metabolic rate during torpor, or the reduction of metabolic rate expressed as percentage of basal metabolism (BMR). Initially, animals were grouped into species displaying either daily torpor or prolonged torpor (hibernation) according to observations from original sources. Both cluster and discriminant analyses supported this division, and further analyses were therefore based on these two groups. Frequency distributions for all tvariables tested differed significantly (P < 0.001) between daily torpor and hibernation. The average maximum torpor bout duration was 355.3 h in hibernators and 11.2 h in daily heterotherms. Mean minimum Tb's were lower in hibernators than in daily heterotherms (5.8° C vs. 17.4° C) as were minimum metabolic rates measured as rate of oxygen consumption (V̇o2; 0.037 vs. 0.535 mL O₂ g⁻¹h⁻¹), and the metabolic rate reduction expressed as percentage of BMR (5.1% vs. 29.5%). Furthermore, mean body weights were significantly higher in hibernators (2384 g) than in daily heterotherms (253 g; P < 0.001). Thus, the comparisons of several physiological variables appear to justify a distinction between the two torpor patterns. However, of all variables tested, only the frequency distributions of maximum torpor bout duration (1.5-22 h for daily torpor; 96-1,080 h for hibernation) showed a clear gap between daily heterotherms and hibernators. The minimum V̇o2 also distinguished clearly between daily heterotherms and hibernators. All other variables, including minimum Tb during torpor, did not show a complete separation between the two patterns of heterothermia. We therefore suggest that classification of torpor patterns should be based on the duration of torpor bouts or the minimum V̇o2 during torpor.